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4Short Report

During 2002, the Space Studies Board and its committees issued one short report, the main text of which this

section presents.

4.1 Review of the Redesigned Space Interferometry Mission (SIM)

On September 12, 2002, Space Studies Board Chair John H. McElroy and Board on Physics and Astronomy

Chair John P. Huchra sent the following letter report to Dr. Edward Weiler, NASAs Associate Administrator for

Space Science.

This letter responds to your request (attachment 1*) that the Committee on Astronomy and Astrophysics (CAA;

see attachment 2*), a joint committee of the Space Studies Board and the Board on Physics and Astronomy, review

the redesigned Space Interferometry Mission (SIM) to assess how its current scientific capabilities compare with

what was originally envisioned by the 1991 Astronomy and Astrophysics Survey Committee (AASC) report1 and

later reaffirmed by the 2001 AASC report.2 The CAA met in Washington, D.C., on May 1-2, 2002, was briefed by

the Jet Propulsion Laboratory (JPL) project team on the redesigned SIM (see attachment 3*), and undertook areview of the redesigned missions scientific capabilities.3

SIM (originally AIM, the Astrometric Interferometry Mission) was prioritized in the 1991 AASC report as the

fourth-ranked space program of moderate class. The science goals specified in the 1991 report were . . . definitive

searches for planets around stars as far away as 500 light-years through the wobbles of the parent star, trigonometric

determination of distances throughout the galaxy, and the study of the mass distributions of nearby galaxies from

stellar orbits (p. 85). The report defined the wide-angle astrometric precision necessary for AIM to achieve the

specified scientific goals: The mission requirement would be to measure positions of widely separated objects to a

visual magnitude of 20 with a precision of 30 millionths of an arcsecond; a more challenging goal would be to

measure positions with a precision of 3 millionths of an arcsecond (p. 85). The report also noted AIMs technology

demonstration potential for future space interferometry missions.

1Astronomy and Astrophysics Survey Committee, National Research Council. 1991. The Decade of Discovery in Astronomy and Astrophysics

(Washington, D.C.: National Academy Press).2Astronomy and Astrophysics Survey Committee, National Research Council. 2001. Astronomy and Astrophysics in the New Millennium

(Washington, D.C.: National Academy Press).3The CAA was not asked for, nor did it attempt to conduct, a technical assessment of the redesigned mission concept; however, two outside

experts in interferometry (David Mozurkewich, Naval Research Laboratory, and Roger Angel, University of Arizona) were invited to the meeting

to provide independent technical expertise during public discussions of the redesigned mission. The CAA was also briefed on the conclusions of

a 2001 NASA external review board report (available at http://sim.jpl.nasa.gov/library/SIM_ERB.pdf) by Harold McAlister, Georgia State

University.

*Attachments 1-3 are not reprinted in this annual report.

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In the intervening decade, the discovery of extrasolar planets opened up an entirely new and exciting scientific

field, providing the impetus for a more ambitious narrow-angle astrometry capability on SIM that would allow the

detection of planets with masses as low as a few times that of Earth. As presented to the 2001 AASC by the JPL

project team during the AASCs deliberations in 1999, SIM would have had a capability for wide-angle astrometry

(10 microarcseconds requirement; 4 microarcseconds goal) and for narrow-angle astrometry (3 microarcseconds

requirement; 1 microarcsecond goal), an astrometry magnitude limit of 20, full synthesis imaging plane coveragefrom 1 to 10 meters, and nulling capability (the last two being precursor technology demonstrations for TPF, the

Terrestrial Planet Finder, which was one of the recommended new major missions in the 2001 report). As the

scientific capabilities were enhanced, and as the technical understanding of the mission matured, the estimated

mission cost increased so that the mission graduated from the astronomy surveys moderate mission category (with

an estimated mission cost of ~$250 million in FY1990 dollars) to the major mission category (with an estimated

mission cost of ~$1,000 million in FY2000 dollars).

The 2001 AASC report reaffirmed the recommendation of the 1991 AASC by endorsing the completion of

SIM, although the 2001 report did not explicitly mention the increased estimated cost of SIM, which had the effect

of moving it from the decadal surveys moderate-mission category to the major-mission category. The scientific

capabilities explicitly called for by the 2001 AASC were . . . [enabling] the discovery of planets much more similar

to Earth in mass and orbit than those detectable now, and . . . [permitting] astronomers to survey the Milky Way

Galaxy 1,000 times more accurately than is possible now (p. 20). The report emphasized the particular attraction

of the dual capability of the new SIM, noting that this capability would enable . . . both narrow-angle astrometry fordetecting planets and wide-angle astrometry for mapping the structure of the Milky Way and other nearby galaxies

(p. 111). The report of the 2001 AASCs Panel on Ultraviolet, Optical, and Infrared Astronomy from Space

(UVOIR Panel),4 which contains more detailed and explicit statements about SIM and its scientific goals than those

included in the main AASC report, stressed that the primary scientific objective of the SIM mission is ultrahigh-

accuracy astrometry (p. 337). SIMs capabilities for making distance measurements of 1 percent accuracy to

distances of several kiloparsecs and of 10 percent accuracy throughout the Galaxy would address the astrophysical

goals of providing a firm foundation for the understanding of stellar astrophysics . . . [making] luminosity

determinations for key classes of stars . . . [to] reduce the calibration uncertainties in the cosmological distance scale

. . . [and refining] our understanding of galactic structure, and in particular the structure of the Galactic halo, thus

tracing the distribution of dark matter (p. 338). The UVOIR Panel stated that [s]earching for planets near stars in

the solar neighborhood is the most ambitious of SIMs goals . . . [because the search] should generate a preliminary

survey of the local planetary population and a more extensive survey of the Jovian-mass planets . . . and . . .

[thereby] not leave any ambiguity about the masses of extrasolar planets (p. 338).To contain rising costs, NASAs Office of Space Science directed in October 2000 that SIM be redesigned. The

simpler (shared-baseline) design selected in May 2001precisely one year after the recommendations of the

AASC were made publicomits the nulling and synthesis imaging intended as technology demonstrations for the

future Terrestrial Planet Finder mission, thus lowering SIMs estimated cost and significantly reducing the technical

risk. Loss of synthesis imaging translates to a loss of SIMs capability to study certain high-surface-brightness

structures, a capability that would have permitted, for example, measurement of the rotation of the inner parts of

galaxies and the diameters of supernova remnants. Loss of nulling translates to loss of SIMs so-called nulling-

imaging capability that would have enabled the study of objects with a large dynamic range in brightness such as

astrophysical processes occurring near a star, for example, winds of Wolf-Rayet stars, circumstellar dust disks of

main sequence stars, and disks of young stellar objects. These imaging and nulling studies are interesting in their

own right, but they are not integral to the core astronomical goals described in the 1991 and 2001 AASC reports,

such as, for example, mapping of galactic structure, calibration of the cosmological distance scale, and detection of

extrasolar planets.

At the May 1, 2002, CAA meeting, the SIM project team from JPL reported that current component

performance suggests the projects ability to meet the 2001 UVOIR Panel reports stated wide-angle requirement of

10 microarcseconds and to exceed the goal (3.2 microarcseconds, compared to the goal of 4 microarcseconds),

making it possible for SIM to address the fundamental wide-angle astrometric science goals emphasized by the

4Panel on Ultraviolet, Optical, and Infrared Astronomy from Space, Astronomy and Astrophysics Survey Committee, National Research

Council. 2001.Astronomy and Astrophysics in the New Millennium: Panel Reports, pp. 327-374 (Washington, D.C.: National Academy Press).

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UVOIR Panel and the 1991 and 2001 AASC reports. SIMs predicted narrow-angle accuracy performance will

meet the UVOIR Panel reports recommended requirement of 3 microarcseconds and is within a factor of two of the

performance goal (1.7 microarcseconds, compared to the goal of 1 microarcsecond). Table 1 summarizes the key

AASC and corresponding JPL expected mission parameters. SIMs predicted narrow-angle capability will allow

the detection of planets with masses five times that of Earth out to a distance of 10 parsecs. Progress toward thenarrow-angle performance goal is continuing, and additional improvement in narrow-angle precision will lead to

greater capability in an area where the SIM contribution will be unique: the measurement of masses and orbits for

extrasolar planets with masses a few times that of Earth. However, the CAA notes that the size of the sample of such

planets that SIM will detect and accurately characterize is very uncertain.

The CAA was pleased to hear the JPL teams report that excellent progress has been made in meeting SIMs

technical goals. The committee concludes that the core astronomical capabilities motivating the original recommen-

dation for SIM in the 1991 AASC report, and the enhanced astronomical and planet-finding capabilities endorsed

by the 2001 AASC report, have been preserved in the current rescope. The CAA reaffirms the scientific excitement

of the 2001 AASC for the important new planet-finding narrow-angle science capability of SIM. Nevertheless, the

CAA is concerned by the JPL redesign plans stated wide-angle accuracy requirement of 30 microarcseconds

which was the level recommended by the 1991 AASC but not by the 2001 AASC. The 2001 recommendation is the

benchmark to which a redesigned SIM should be compared. Accordingly, the CAA reaffirms the following

statement made in the 2001 UVOIR Panel report: The majority of exciting astrophysical goals already pose achallenge under the floor requirements currently proposed: 3 microarcseconds for narrow-angle astrometry and 10

microarcseconds for wide-angle astrometry. Relaxing these floor requirements might eliminate large fractions of

the important science. If SIM is unable to meet these requirements within its presently planned budget and schedule,

it should be reevaluated by the scientific community to determine if it should remain a high priority as a major

mission (p. 338). Specifically, the CAA notes that not meeting the wide-angle astrometry requirement recom-

mended by the 2001 AASC report would preclude the ability to accurately measure the distances to, and ages of,

globular clusters, and to pursue almost all of the fundamental galactic structure science, such as the accurate

measurement of the distance from Earth to the center of the galaxy, the shape of the galaxys outer rotation curve,

and other basic properties of the Milky Way.

In conclusion, the CAA finds that the scientific capabilities (based on the current component analysis) of the

redesigned SIM meet the core capabilities originally envisioned by the 1991 AASC report and reaffirmed by the

2001 AASC report. In making this assessment, the CAA compared the reported astrometric performance of the

redesigned SIM with the performance requirements and goals stated in the report of the 2001 AASCs UVOIR

Panel, and not with the less exacting wide-angle requirement level stated in the decade-old 1991 AASC report as

was apparently expected by the JPL project team based on its May 2002 presentation to the CAA. This distinction

should not matter in practice since (1) the JPL team reported that the wide- and narrow-angle capabilities are

coupled in such a way that the wide-angle requirement/goal will be met on the road to meeting the more demanding

narrow-angle requirement/goal, and (2) the JPL team reported an expected performance better even than the

performance goal accuracy recommended by the 2001 AASC. Nevertheless, if the developing SIM project fails at

some future time to meet the 2001 AASC performance requirements, it should be reevaluated by the scientific

community to determine if it should remain a high-priority astronomy and astrophysics mission.

Table 1 Key SIM Parameters Defined by AASC Reports and JPLs SIM Project Team

Wide-angle Astrometry Narrow-angle Astrometry

Requirement Goal Requirement Goal Magnitude

Concept (s) (s) (s) (s) Limit (V) Nulling? Synthesis Imaging?

1991 AASC (AIM) 30 3 20 No No

2001 AASC (SIM) 10 4 3 1 20 Yes Full UV plane from

1 to 10 m

2002 JPL redesign plan 30 4 3 1 20 No 10-m baseline only

(plus rotation)

Reported current 3.2 1.7

performance

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The CAA notes that it did not conduct a thorough assessment of the impact of current and future ground- and

space-based astrometry missions on the science enabled by SIM, so no discussion of these other capabilities is

included in this letter. In addition, while the current assessment has focused on an evaluation of the extent to which

the mission can still meet the scientific objectives considered by the 1991 AASC and assumed by the 2001 AASC,

the CAA did not address the question of whether the redesigned SIM would be given high ranked-priority in the

context of the major missions recommended by the 2001 AASC report. As a result, neither a previous decadalsurvey committee nor the CAA has ever explicitly priority-ranked SIM against other major (as opposed to

moderate-cost) astronomy and astrophysics missions. Finally, since the CAA conducted a scientific, not a technical,

assessment of the mission, the committee expects that the SIM project will continue to be peer reviewed by non-

advocate panelsas is standard NASA practice.

Signed by

John H. McElroy

Chair, Space Studies Board

John P. Huchra

Chair, Board on Physics and Astronomy